1. Field of the Invention
This application relates generally to digital image signal processing, and more particularly to the compression of interlaced motion images.
2. Description of the Related Art
Image and motion image signals are increasingly being processed in the digital domain. One set of problems encountered in digital image signal processing result from signal formats used in conventional analog video processing. For example, certain formats, particularly conventional broadcast television, implement signal interlacing. In video interlacing, a frame is comprised of two separate rasters. A first raster comprises a series of lines in the frame, and a second raster comprises scan lines that reside between the lines from the first raster. The first and second rasters can respectively be referred to as first and second fields. Thus the video signal can be said to include a series of frames, with each frame including first and second interlaced fields.
There are various problems introduced by interlaced motion images and the conventional processing methods. For example, interlaced images may have relative motion between the rasters, which does not correlate well vertically. One solution to this could be to encode images with significant motion by frame. However, this would result in very inefficient encoding.
Alternatively, images can be processed by field to allow for poor vertical correlation in the case of motion. However, if this is done, the encoding advantage of any high correlation in still areas is lost, again resulting in inefficient encoding. Additionally, if images are processed by field, slight variations in DC errors or quantifier linearity will cause horizontal stripes in a motion free fame viewed as a still.
Typically, block based compression schemes (such as MJPEG and MPEG) treat the interlaced video decorrelation problem on a block level. Thus, each block in a frame is classified as a motion block or a still block. The still image blocks are processed differently from the motion blocks.
There are various problems with the block based approach. Particularly, block based motion analysis can generate boundaries and visual artifacts due to incorrect motion estimation decisions. Also, block based motion analysis can generate very large local errors, making it unsuitable for applications where local error bounds are desired, as in medical, technical or critical production applications. Finally, the block based approach can cause an obvious discontinuity or defect where an object in motion spans adjacent blocks, particularly where one block is still processed while an adjacent block is motion processed.
Thus, there remains a need for digital image signal processing, particularly processing incorporating lossy compression, that addresses the problems introduced by frames having interlaced fields.